The invention relates to a method and an apparatus for determining the mass of portioned units of active substances, in particular capsules, tablets or dragees, in particular in the pharmaceutical industry.
In the case of portioned units of active substances, it is important to monitor whether each unit of active substances actually has the desired mass. As a result, it is possible for example to determine whether a casing has been filled incompletely or not at all. A customer would rightly be dissatisfied if the units of active substances acquired by him did not contain the quantity of active substances which he expected. This applies in particular to the pharmaceutical sector, where accurate metering of the units of active substances matters.
In the pharmaceutical industry, the pharmaceutical active substances, together with specific filler substances such as starch, lactose and so on, are pressed in tablet form by specific filling machines or put into gelatin-like capsules or formed as dragees. The corresponding machines reach high production speeds with filling rates of less than 50 000 to more than 300 000 per hour. It is critical to the effectiveness of the pharmaceutical preparation that the unit of active substances contains the necessary quantity of active substances and this is also actually maintained at the high filling rates. The volume contents of the units of active substances in the pharmaceutical sector vary between 800 mg in the case of large capsules down to 50 mg or, in the case of very small doses, down to 5 mg.
It is known to determine or to monitor the masses of the units of active substances produced by means of weighing (DE 198 19 395 C1). Because of the large number of units of active substances per unit time, and on account of the fact that the mechanical weighing operation naturally needs a certain amount of time, it is possible either only for random samples to be removed and weighed and/or it is necessary for a large number of devices arranged in parallel to be provided for the weighing. Using random samples, however, only the general quality of the units of active substances produced can be checked. Individual checking is not possible, and so deviations of individual samples from the average quality cannot be monitored and removed. On the other hand, the weighing of each individual unit of active substances by means of weighing devices arranged in parallel is very complicated. Even if, for example, 20 weighing devices were to be used, at the rates specified above, each weighing device would still have to weigh 15 000 units of active substances per hour, that is to say around 4 per second, which still signifies great technical difficulties. The individual units of active substances have to be stopped mechanically on the weighed goods receiver at these production speeds and accelerated beforehand and afterwards. The jolting and abrupt movements lead to considerable mechanical stressing of the units of active substances. Furthermore, in particular in the case of units of active substances provided with casings and with small filling masses, the disadvantage arises that the statistical fluctuations of the mass of the casing are reflected completely as measurement errors of the filled weight. A separate, twofold weighing of the same unit of active substances with and without filling could rule out this error, but, in the case of weighing technology being used, this increases the problems still further. In addition, as a result of the sensitive mechanical guidance in these balances, if there were a change in the format of the units of active substances to be weighed, considerable mechanical conversion of product guides would be necessary.
Attempts have also been made to overcome the above disadvantages, which are brought about by the inertia of the weighing technology, by means of capacitive measuring techniques (U.S. Pat. No. 4,223,751 A, U.S. Pat. No. 5,602,485 A, DE-A 29 39 406). With this method, with units of active substances moved at high speed through the capacitor, a mass-proportional electric signal can be determined only if the moisture content of the units of active substances and of the filler material remains exactly constant. As a result of slight fluctuations of the water content of the active substance in the units of active substances, because of the high dielectric constant of the water, a disproportionate change in the mass signal is produced. A small change in the moisture thus produces a large deviation in the mass signal from the actual mass.
An object of the invention is to provide a method and an apparatus of the type mentioned at the beginning with which the mass of the units of active substances can be determined reliably, accurately and quickly without excessive expenditure.